From that fount of information -- lots of it quite wrong -- Wikipedia:
"Provel is a white processed cheese that is popular in St. Louis, Missouri. Provel is produced with cheddar, Swiss, and provolone. It is soft at room temperature, with a gooey and almost buttery texture, and thus has a low melting point. It is the traditional topping for St. Louis-style pizza. It is also often served on salads, chicken and the Gerber sandwich."
Sounds sort of like a low-grade Velveeta to me.